The prior art is illustrated by the following publications some of which will be mentioned by way of reference in the description hereafter.                Clark, R. K. and K. L. Bickham: “A mechanistic model for cuttings transport”, SPE No.28306, 69th Annual Technical Conference and Exhibition, New Orleans, 25-28 September 1994,        Fleury, M.: “Validity of permeability prediction from NMR measurements ”, No. GERM Annual Meeting, La Pommeray, France, 14-18 May 2001,        Godefroy, S.: “Etudes RMN de la dynamique des molécules aux interfaces solide-liquide: des matériaux poreux calibres aux roches réservoirs ”, Thése de l'Ecole Polytechnique, 2001,        Kenyon, W. E.: <<A three-part study of NMR longitudinal relaxation properties of water saturated sandstones >>, SPE Formation Evaluation, March, pp. 622-636, 1989,        Logan, W. D., J. P. Horkowitz, R. Laronga et al.: <<Practical Application of NMR logging in carbonate reservoirs >>, SPE Reservoir Eval. & Eng., October, pp. 438-448, 1998,        Naegel, M., E. Pradié, T. Delahaye et al. <<Cuttings flow meters monitor hole cleaning in extended reach wells >>, SPE No. 50677, European petroleum conference, The Hague, The Netherlands, 20-22 October 1998,        Nigh, E. and M. Taylor: “Wellsite determination of porosity and permeability using drilling cuttings ”, C. W. -1. Society 10th Formation Evaluation Symposium, 1985,        Pilehvari, A. A., J. J. Azar and S. A. Shirazi: “State-of-the-art cuttings transport in horizontal wellbores ”, SPE No. 37079, International conference on horizontal well technology, Calgary, Canada, 18-20 November 1996,        Santarelli, F. J., A. F. Marsala, M. Brignoli et al.: <<Formation evaluation from logging on cuttings >>, SPE Reservoir Eval. & Eng., June, pp. 238-244, 1998,        Seevers, D. O.: “A nuclear magnetic method for determining the permeability of sandstones ”, No. SPWLA 1966, or        Timur, A.: “An investigation of permeability, porosity and residual water saturation relationships ”, No. SPWLA 1968.I-1 Existing Petrophysical Measurements from Rock Fragments        
Measurement of petrophysical parameters such as permeability, porosity and capillary properties on rock fragments taken up to the surface during drilling of a well through an underground formation constitutes an interesting opportunity for operator companies to obtain rapidly a first petrophysical characterization of producing zones crossed through by the well.
Patent FR-2,809,821 filed by the applicant describes a system for evaluating physical parameters such as the absolute permeability of porous rocks in a zone of an underground formation, from cuttings carried up to the surface with the drilling mud. In an enclosure where the cuttings are immersed in a viscous fluid, some of this fluid is injected at a pressure increasing with time, up to a predetermined pressure threshold, so as to compress the gas trapped in the pores of the rock. This injection phase is followed by a relaxation stage with injection stop. The evolution of the pressure during the injection process having been modelled from initial values selected for the physical parameters of the cuttings, a computer adjusts them iteratively so as to allow the modelled pressure curve to best coincide with the pressure curve really measured.
Patent applications FR-02/0,023 and FR-03/03,742 filed by the applicant describe another method of evaluating physical parameters such as the absolute permeability and the porosity of rocks in a zone of an underground formation, also from cuttings. An enclosure containing the rock fragments and filled with a viscous fluid is communicated with a tank containing this fluid at a predetermined pressure so as to compress the gas trapped in the pores of the rock. The application time of this pressure, according to whether it is short or long, allows to measure either the pressure variation in the enclosure or the variation of the volume effectively absorbed by the rock fragments. Then, the evolution of the pressure or of the volume in the enclosure is modelled from initial values selected for the physical parameters of the rock fragments so that the modelled evolution best adjusts with the measured evolution of the physical parameter in the enclosure.
I-2 Use of NMR in the Petroleum Industry
The non-intrusive measuring technique referred to as NMR (Nuclear Magnetic Resonance) has been known for a long time. It has aroused a very strong interest in the petroleum industry twenty years ago thanks to the advances made in data acquisition and processing. The principle of this technique is described in many references, notably by Seevers 1966, Timur 1968, Kenyon 1989 or Godefroy 2001, already mentioned above. To present things in a very simplified way, measurement consists in first exciting the protons along a magnetic field imposed by the device, then in letting the protons come back to their initial state (relaxation). In porous media, the magnetic relaxation signal obtained depends on the nature of the fluids contained in the rock, and on the pore geometry. For a proton, relaxation (T2) is all the faster as it is located in a low-extension pore. T2 is related to surface relaxivity P2, surface area S and volume V by the relation:
      1          T      2        =            ρ      2        ⁢          S      V      
I-2.1 Porosity Evaluation
When the nature of the fluid contained in the pores is known, the amplitude of the NMR signal can be converted into volume. This allows to directly evaluate the amount of fluid contained in the rock, i.e. directly the difference between the total volume of rock and the solid volume (Vt−Vs). If the total volume is known, the value of the porosity can be deduced. In a more laboratory-oriented context, the total volume has to be measured separately by means of a powder pycnometer for example, or from the dimensions of the sample (core).
  ϕ  =                    V        t            -              V        s                    V      t      
NMR measuring devices are widely used for logging. In this context, the total volume corresponds to the measurement volume of these devices (corrected for edge effects).
I-2.2 Permeability Evaluation
Various empirical correlations have been proposed for evaluating the value of the permeability from the distribution characteristics of T2. NMR logging has been presented as a predictive tool for measuring the permeability in a well. This technique has also been considered for measurements on cuttings since it is fast and flexible, in the field as well as in the laboratory. The models are based on the use of measuring times T1 and T2 which represent the respectively longitudinal and transverse relaxation times from the NMR signal. A synthesis of the existing models is given in the aforementioned reference Fleury et al. (2001). In the following publications, also mentioned above, permeability k is modelled by the empirical relations:
                                          Seevers            ⁢                                                  ⁢                          (              1966              )                        ⁢                                                  ⁢            k                    =                                    C              ⁡                              (                                  1                  -                                      S                    wi                                                  )                                      ⁢                          T              1              2                        ⁢            ϕ                          ⁢                                  ⁢                              Timur            ⁢                                                  ⁢                          (              1968              )                        ⁢                                                  ⁢            k                    =                      C            ⁢                                                  ⁢                          1                              S                wi                2                                      ⁢                          ϕ              4.4                                      ⁢                                  ⁢                              Kenyon            ⁢                                                  ⁢                          (              1988              )                        ⁢                                                  ⁢            k                    =                                    CT                              2                ⁢                ML                            2                        ⁢                          ϕ              4                                                                                                                                              k        =                  C          ⁢                                          ⁢                      1                          S              wi              2                                ⁢                      ϕ            4.4                              
In the previous models, the value of Swi is obtained from the signal of T2 by considering the volume which corresponds to short relaxation times (small pores) under a certain cutoff threshold. In the literature, the default values of this cutoff threshold are 33 ms for sandstones in contrast to 100 ms for carbonates (purely empirical values). FIG. 1 illustrates the use of the cutoff threshold. All these expressions were successfully validated on a large number of samples belonging to the same petrophysical group.
When changing petrophysical groups, the prediction quality can decrease significantly. This results from the NMR measurement, which is not a direct permeability measurement. Thus, more general relations relating the permeability to the NMR signal are preferably used:
                              k          =                      C            ⁢                                                  ⁢                          T              2              a                        ⁢                          ϕ              b                                      ,                            k        =                  C          ⁢                                          ⁢                      1                          S              wi              a                                ⁢                      ϕ            b                              where C, a and b Depend on the Porous Structure.
Parameters C, a and b thus have to be adjusted to obtain satisfactory predictions for each rock.
I-3 NMR Measurements on Cuttings
In the laboratory
A NMR measurement technique allowing to obtain an evaluation of the porosity of cuttings obtained by crushing core samples of known properties is outlined very roughly by Santarelli et al. (1998) mentioned above. Only the comparison with the reference measurements is given. A good agreement is obtained with the reference measurements, but a certain degradation is observed in relation to the gas (helium) expansion technique which is translated into a lower correlation coefficient (R2=0.87) and a greater mean deviation in relation to the first bisectrix (of the order of 3-4 porosity units).
In the Field
The use of a field NMR device allowing to determine the porosity and the permeability from cuttings short after they have reached the surface is described by Nigh et al. (1985). The first measuring stage consists in properly identifying the cuttings recovered in terms of depth (lag time calculation). It is recommended to frequently inject tracers into the mud in order to contribute towards readjustment. For a drilling operation carried out with water-base mud, the cuttings are first cleaned in a washing cell filled with 3% NaCl brine. The operation is repeated until the cuttings are clean. If an oil-base mud is used, the cuttings are cleaned with solvents. The equivalent of two teaspoonfuls is necessary for one measurement. Once the cleaning operation complete, the cuttings are examined to check that they really come from the reservoir levels corresponding to the lag time. The cuttings selected are then fed into a cell filled with water and placed under vacuum so as to completely saturate them with liquid. The cuttings are thereafter removed from the cell and contacted with a ceramic to remove the water from the surface, and the total volume of rock is calculated. The NMR measurement allows to directly determine the value of the porosity and of the irreducible water saturation Swi. The results obtained show that the porosity measurement systematically underestimates the results of the measurements on core samples and the logs. The authors attribute these differences to depth readjustment problems and to the fact that the cuttings give a value corresponding to a precise depth whereas there is an averaging effect with the logging tools (30 cm on average). According to the saturation procedure described in the publication (which is not the one recommended in the laboratory: API standards), it is also possible that part of the pore network has remained gas-saturated, which could explain the underestimation observed.